arXiv subjects

F. Lebrun

Publications and source records attributed to F. Lebrun.

At least 55 records · Page 3Linked to original sources

arXiv2005

Global characteristics of the first IBIS/ISGRI catalogue sources: unveiling a murky episode of binary star evolution

INTEGRAL is the first gamma-ray astronomy mission with a sufficient sensitivity and angular resolution combination appropriate to the detection and identification of considerable numbers of gamma-ray emitting sources. The large field of view enables INTEGRAL to survey the galactic plane on a regular (~weekly) basis as part of the core programme. The first source catalogue, based on the 1st year of core programme data has been completed and published (Bird et al., 2004). It contained 123 gamma-ray sources (24 HMXB, 54 LMXB, 28 ``unknown'', plus 17 others) - sufficient numbers for a reasonable statistical analysis of their global properties. The detection of previously unknown gamma-ray emitting sources generally exhibiting high intrinsic absorption, is intriguing. The substantial fraction of unclassified gamma-ray sources suggests they must constitute a significant family of objects. In this paper we review the global characteristics of the known galactic sources as well as the unclassified objects. We present Log(N)-Log(S) distributions, angular distributions, and for systems with reliable distance estimates the spatial distributions within the Galaxy and luminosity functions. For the unknown sources, this statistical analysis has shown that they are most likely to be HMXBs containing a highly magnetised neutron star. The lack of X-ray counterparts for these sources indicates a high degree of intrinsic obscuration.

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arXiv2005

INTEGRAL IGR J18135-1751=HESS J1813-178: A new cosmic high energy accelerator from keV to TeV

We report the discovery of a soft gamma ray source, namely IGR J18135-1751, detected with the IBIS imager on board the INTEGRAL satellite. The source is persistent and has a 20-100 keV luminosity of $\sim$5.7 $\times$ 10$^{34}$ erg s$^{-1}$ (assuming a distance of 4kpc). This source is coincident with one of the eight unidentified objects recently reported by the HESS collaboration as part of the first TeV survey of the inner part of the Galaxy. Two of these new sources found along the Galactic plane, HESS J1813-178 and HESS J1614-518, have no obvious lower energy counterpart, a fact that motivated the suggestion that they might be dark cosmic ray accelerators. HESS J1813-178 has a strongly absorbed X-ray counterpart, the ASCA source AGPS273.4-17.8, showing a power law spectrum with photon index $\sim$ 1.8 and a total (galactic plus intrinsic) absorption corresponding to N$_H$ $\sim$5 $\times$ 10$^{22}$ cm$^{-2}$. We hypothesize that the source is a pulsar wind nebula embedded in its supernova remnant. The lack of X/gamma-ray variability, the radio morphology and the ASCA spectrum are all compatible with this interpretation. In any case we rule out the hypothesis that HESS J1813-178 belongs to a new class of TeV objects or that it is a cosmic "dark particle" accelerator.

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arXiv2004

Hard X-ray Emission Clumps in the gamma-Cygni Supernova Remnant: an INTEGRAL-ISGRI View

Spatially resolved images of the galactic supernova remnant G78.2+2.1 (gamma-Cygni) in hard X-ray energy bands from 25 keV to 120 keV are obtained with the IBIS-ISGRI imager aboard the International Gamma-Ray Astrophysics Laboratory INTEGRAL. The images are dominated by localized clumps of about ten arcmin in size. The flux of the most prominent North-Western (NW) clump is (1.7 +/- 0.4) 10^{-11} erg/cm^2/s in the 25-40 keV band. The observed X-ray fluxes are in agreement with extrapolations of soft X-ray imaging observations of gamma-Cygni by ASCA GIS and spatially unresolved RXTE PCA data. The positions of the hard X-ray clumps correlate with bright patches of optical line emission, possibly indicating the presence of radiative shock waves in a shocked cloud. The observed spatial structure and spectra are consistent with model predictions of hard X-ray emission from nonthermal electrons accelerated by a radiative shock in a supernova interacting with an interstellar cloud, but the powerful stellar wind of the O9V star HD 193322 is a plausible candidate for the NW source as well.

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arXiv2004

SPI Measurements of the Diffuse Galactic Hard X-ray Continuum

INTEGRAL Spectrometer SPI data from the first year of the Galactic Centre Deep Exposure has been analysed for the diffuse continuum from the Galactic ridge. A new catalogue of sources from the INTEGRAL Imager IBIS has been used to account for their contribution to the celestial signal. Apparently diffuse emission is detected at a level ~10% of the total source flux. A comparison of the spectrum of diffuse emission with that from an analysis of IBIS data alone shows that they are consistent. The question of the contribution of unresolved sources to this ridge emission is still open.

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arXiv2004

From BeppoSAX to INTEGRAL: PDS observations of hard X-ray sources detected in the IBIS survey

The first IBIS galactic plane survey has provided a list of high energy emitting objects above 20 keV; these sources have been detected mostly in the crowded region of the Galactic Centre and partly along the Galactic Plane. In order to validate the detection procedure, to help in the identification process and to study the nature of these IBIS sources, this list has been cross correlated with the data archive of the PDS instrument on BeppoSAX, which operated in a similar energy band and with a similar sensitivity. We discover a number of associations whose detailed analysis will be particularly useful for the survey work. Also, thanks to the imaging capability of IBIS/ISGRI, objects which could not be studied by the PDS due to contamination from nearby sources can now be associated with a definite source or sources.

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arXiv2004

High-energy emission from the stellar wind collisions in gamma-2 Velorum

The binary system gamma-2 Velorum (WC8+O7.5) contains the nearest known Wolf-Rayet star to the Sun, at a distance of 258$_{-31}^{+41}$ pc. Its strong radio emission shows evidence for a partially absorbed nonthermal component, which has been interpreted as synchrotron emission from electrons accelerated in the colliding wind region. Inverse Compton cooling of these electrons in the intense UV radiation field from the O-type companion star could produce a significant hard X-ray and gamma-ray emission, whose flux depends on the ratio of the energy densities of magnetic to seed photon fields. The Vela region was observed with the INTEGRAL satellite in 2003, as part of the Core Programme. No signals from gamma-2 Vel are detected in the images obtained with the IBIS/ISGRI coded aperture instrument in the energy ranges 20-40 and 40-80 keV. From the derived 3$σ$ upper limits, we show that the average magnetic field near the region of stellar wind collision should be relatively high, greater than about 1 G. The high-energy emission of gamma-2 Vel might be detected with the forthcoming GLAST experiment.

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arXiv2003

ISGRI: the INTEGRAL Soft Gamma-Ray Imager

For the first time in the history of high energy astronomy, a large CdTe gamma-ray camera is operating in space. ISGRI is the low-energy camera of the IBIS telescope on board the INTEGRAL satellite. This paper details its design and its in-flight behavior and performances. Having a sensitive area of 2621 cm$^2$ with a spatial resolution of 4.6 mm, a low threshold around 12 keV and an energy resolution of $\sim$ 8% at 60 keV, ISGRI shows absolutely no signs of degradation after 9 months in orbit. All aspects of its in-flight behavior and scientific performance are fully nominal, and in particular the observed background level confirms the expected sensitivity of 1 milliCrab for a 10$^6$s observation.

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arXiv2003

In flight calibration of the ISGRI camera

ISGRI, the IBIS low energy camera (15 keV - 1 MeV) on board INTEGRAL, is the first large CdTe gamma-ray imager in orbit. We present here an overview of the ISGRI in-flight calibrations performed during the first months after launch. We discuss the stability of the camera as well as the CdTe pixels response under cosmic radiation. The energy calibra tions were done using lead and tungsten fluorescence lines and the $\mathrm{^{22}Na}$ calibration unit. Thermal effects and charge correction algorithm are discussed, and the resulting energy resolution is presented. The ISGRI background spatial and spectral non-uniformity is also described, and some image correction results are presented.

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arXiv2003

INTEGRAL discovery of a bright highly obscured galactic X-ray binary source IGR J16318-4848

INTEGRAL regularly scans the Galactic plane to search for new objects and in particular for absorbed sources with the bulk of their emission above 10-20 keV. The first new INTEGRAL source was discovered on 2003 January 29, 0.5 degree from the Galactic plane and was further observed in the X-rays with XMM-Newton. This source, IGR J16318-4848, is intrinsically strongly absorbed by cold matter and displays exceptionally strong fluorescence emission lines. The likely infrared/optical counterpart indicates that IGR J16318-4848 is probably a High Mass X-Ray Binary neutron star or black hole enshrouded in a Compton thick environment. Strongly absorbed sources, not detected in previous surveys, could contribute significantly to the Galactic hard X-ray background between 10 and 200 keV.

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arXiv2003

IBIS preliminary results on Cygnus X-1 spectral and temporal characteristics

We report preliminary results of a broadband spectral and temporal study of the black-hole binary Cyg X-1 performed with the IBIS telescope. Cyg X-1 was the first pointed celestial target of IBIS during the INTEGRAL Performance and Verification Phase, 2002 Nov.-Dec., for a total observing time of 2 Ms in both staring and dithering mode. Here, we report on only the staring, on-axis, observation performed in a stable instrument configuration. During the observing period the source was in its characteristic low/hard state, in which a few flares and dips have been detected. The IBIS/ISGRI results demonstrate that the INTEGRAL observatory offers a unique capability for studying correlations between hardness and/or flux in different bands over a wide photon energy range. One of our new results is finding that the hardness-flux correlation changes the sign twice over the 20-220 keV; first from positive to negative at 50 keV, and then back to positive at 120 keV. The former change appears to be due to the spectral curvature introduced by variable Compton reflection. The latter may be due spectral pivoting.

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arXiv2003

IBIS performances during the Galactic Plane Scan - I. The Cygnus region

The Plane of our Galaxy is regularly observed by the INTEGRAL satellite by means of scheduled scans. We present here results from the IBIS/ISGRI instrument using data from the first two Galactic Plane Scans performed at the end of the Performance Verification phase, while INTEGRAL was pointed towards the Cygnus region. Considering the predicted IBIS sensitivity, we expected three persistent sources to be detectable: Cyg X--1, Cyg X--3, Cyg X--2, in order of decreasing intensity in the hard-X energy range (>15 keV). In addition to these sources, our analysis revealed two more transient sources, confirming the unprecedented IBIS sensitivity. For each exposure (~2200 s) of the two scans, we report on detected source fluxes, variabilities and localisation accuracies.

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arXiv2003

First Results from the IBIS/ISGRI Data Obtained During the Galactic Plane Scan. II. The Vela Region

We report on INTEGRAL/IBIS observations of the Vela region during a Galactic Plane Scan (hereafter GPS) presenting the IBIS in-flight performances during these operations. Among all the known sources in the field of view we clearly detect 4U 0836-429, Vela X-1, Cen X-3, GX 301-2, 1E 1145.1-6141, and H0918-549 in the 20-40 keV energy range. Only Vela X-1 and GX 301-2 are detected in the 40-80 keV energy range, and no sources are visible above. We present the results of each individual observation (~2200 s exposure), as well as those from the mosaic of these scans.

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arXiv2003

First results from the INTEGRAL Galactic plane scans

Scans of the Galactic plane performed at regular intervals constitute a key element of the guaranteed time observations of the INTEGRAL observing programme. These scans are done for two reasons: frequent monitoring of the Galactic plane in order to detect transient sources, and time resolved mapping of the Galactic plane in continuum and diffuse line emission. This paper describes first results obtained from the Galactic plane scans executed so far during the early phase (Dec 2002 - May 2003) of the nominal mission.

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arXiv2003

INTEGRAL and XMM-Newton observations of the weak GRB 030227

We present INTEGRAL and XMM-Newton observations of the prompt gamma-ray emission and the X-ray afterglow of GRB030227, the first GRB for which the quick localization obtained with the INTEGRAL Burst Alert System (IBAS) has led to the discovery of X-ray and optical afterglows. GRB030227 had a duration of about 20 s and a peak flux of 1.1 photons cm^-2 s^-1 in the 20-200 keV energy range. The time averaged spectrum can be fit by a single power law with photon index about 2 and we find some evidence for a hard to soft spectral evolution. The X-ray afterglow has been detected starting only 8 hours after the prompt emission, with a 0.2-10 keV flux decreasing as t^-1 from 1.3x10e-12 to 5x10e-13 erg cm^-2 s^-1. The afterglow spectrum is well described by a power law with photon index 1.94+/-0.05 modified by a redshifted neutral absorber with column density of several 10e22 cm^-2. A possible emission line at 1.67 keV could be due to Fe for a redshift z=3, consistent with the value inferred from the absorption.

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arXiv2001

SIGMA Observations of the Bursting Pulsar GRO J1744-28

We present the results of the GRANAT/SIGMA hard X-/soft gamma-ray long-term monitoring of the Galactic Center (GC) region concerning the source GRO J1744-28, discovered on 1995 Dec. 2 by CGRO/BATSE. SIGMA observed the region containing the source in 14 opportunities between 1990 and 1997. In two of these observing sessions, corresponding to March 1996 and March 1997, GRO J1744-28 was detected with a confidence level greater than 5(sigma) in the 35-75 keV energy band without detection in the 75-150 keV energy band. For the other sessions, upper limits of the flux are indicated. The particular imaging capabilities of the SIGMA telescope allow us to identify, specifically, the source position in the very crowded GC region, giving us a mean flux of (73.1 +/- 5.5)E-11 and (44.7 +/- 6.4)E-11 ergs cm^-2 s^-1 in the 35-75 keV energy band, for the March 1996 and March 1997 observing sessions, respectively. Combining the March 1997 SIGMA and BATSE observations, we found evidence pointing to the type-II nature of the source bursts for this period. For the same observing campaigns, spectra were obtained in the 35 to 150 keV energy band. The best fit corresponds to an optically thin thermal Bremsstrahlung with F(50 keV)=(3.6 +/- 0.6)E-4 phot cm^-2 s^-1 keV^-1 and kT(Bremss)=28 +/- 7 keV, for the first campaign, and F(50 keV)=(2.3 +/- 0.7)E-4 phot cm^-2 s^-1 keV^-1 and kT(Bremss)=18 (+12/-7) keV, for the second. This kind of soft spectrum is typical of binary sources containing a neutron star as the compact object, in contrast to the harder spectra typical of systems containing a black hole candidate

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arXiv2001

Integral/IBIS Observations of persistent black hole spectral states during the Core Program

The Imager on Board the Integral Satellite (IBIS) is one of the two main telescopes of Integral, the ESA soft gamma-ray mission to be launched in 2002. The Integral Core Program will be divided into two main parts, the Galactic Centre Deep Exposure program and the Galactic Plane Scan for a total amount of around 6 .6 10^6 seconds each year. In this paper, we will study the visibility of persist ent galactic black holes as observed by IBIS during these two phases of the Core Program. We will also present what information may be derived from the IBIS observations of spectral/temporal properties (variation of the spectral index, presence/absence of a spectral break, ...) of these binary systems in different spectral states, in particular in the framework of the bulk motion Comptonization model.

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arXiv2001

Observation of a Black-Hole X-ray Nova in Outburst with INTEGRAL

We simulate the observation of a bright Nova Musca-like X-ray nova during outburst with INTEGRAL, the next ESA gamma-ray space observatory. We will show how performances of the INTEGRAL instruments allow deep study of X-ray Novae and will evaluate the scientific output that INTEGRAL will provide on this class of transient gamma-ray sources, which are now believed to contain black holes in low mass binary systems. The variable high-energy feature around 511 keV observed from X-ray Nova Musca in 1991 by the SIGMA telescope would be detected by INTEGRAL at very high significance level. INTEGRAL data will permit to set important constraints on the models and allow to distinguish between electron-positron or nuclear de-excitation origin of the line. Characteristic spectral and timing features detected by INTEGRAL instruments over a very large energy band will also provide clues to understand physics of accretion in these black holes binaries and in particular to distinguish between thermal and non-thermal origin of radiation and to assess the role of bulk motion comptonization.

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arXiv1999

Hard X-Rays from the Galactic Nucleus: Present and Future Observations

In spite of increasing evidences of the presence of a massive Black Hole at the Galactic Center, its radio counterpart, Sgr A*, shows little activity at high energies, and recent models involving energy advection (ADAF) have been proposed to explain this difficulty. We present results on the hard X-ray emission from the galactic central square degree obtained from the SIGMA/GRANAT 1990-1997 survey of this region. The best upper limits available today on the Sgr A* 30-300 keV emission are presented and compared to X-ray data and to the predictions of ADAF models. We also present simulations of Sgr A* observations with the future ESA gamma-ray Observatory INTEGRAL, which show that the imager instrument (20 keV-10 MeV) will be able either to detect the expected ADAF emission from the accreting Galactic Nucleus black hole or set tight upper limits which will constrain the physical parameters of such system.

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